Method of forming a lead-free bump and a plating apparatus therefor

ABSTRACT

The present invention relates to a lead-free bump with suppressed formation of voids, obtained by reflowing a plated film of Sn—Ag solder alloy having an adjusted Ag content, and a method of forming the lead-free bump. The lead-free bump of the present invention is obtained by forming an Sn—Ag alloy film having a lower Ag content than that of an Sn—Ag eutectic composition by plating and reflowing the plated alloy film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lead-free bump and a method offorming the same, and more particularly to a lead-free bump withsuppressed formation of voids, obtained by reflowing a plated film ofSn—Ag solder alloy having an adjusted Ag content, and a method offorming the lead-free bump, and also to a plating apparatus for formingsuch a lead-free bump.

2. Description of the Related Art

In surface mounting technology of semiconductor devices or the like, itis very important to carry out soldering with high reliability. Althoughan eutectic solder containing lead (Sn:Pb=63:37) has heretofore beenused widely in soldering, in the light of environmental contaminationand because of the problem of α-rays generation from lead, developmentof lead-free soldering is under way.

For example, lead-free soldering by means of printing or electroplatingis being studied. With printing, however, there is a limit in itsapproach to fine pitches through the use of a metal mask. Electroplatingis therefore becoming mainstream, for example, for the formation ofwafer bumps.

In the case of forming wafer bumps by electroplating, a heatingoperation (reflowing) is usually carried out to make plated films intothe form of balls. The reflow temperature is preferably as low aspossible in order to avoid thermal damage to other parts that exist inthe substrate. From this viewpoint, many developments of solder alloyshave been directed to making the composition of an alloy closestpossible to the eutectic composition of the alloy in order to make useof the eutectic point.

However, the formation of bumps by electroplating has the problem thatupon reflowing of bumps, voids can be formed in the bumps. The formationof voids is particularly marked with bumps of an Sn—Ag alloy, loweringthe reliability of the bumps.

SUMMARY OF THE INVENTION

There is, therefore, a need for the development of a means to form alead-free Sn—Ag bump by electroplating without formation of voids uponreflowing, and it is an object of the present invention to provide suchmeans.

As a result of studies to obtain a lead-free bump without formation ofvoids, it was discovered by the present inventors that when forming abump by Sn—Ag solder alloy plating, the Ag content in the plated filmhas a great influence on the formation of voids. In particular, voidscan be formed in a lead-free Sn—Ag bump upon reflowing when the Agcontent of the bump is approximately equal to or higher than the Agcontent of the Sn—Ag eutectic composition. As a result of further study,it has now been found that in order to securely prevent the formation ofvoids in a bump of Sn—Ag solder alloy, it is necessary to form the bumpwith a plated alloy film having a lower Ag content than that of theSn—Ag eutectic composition (weight ratio Sn:Ag=96.5:3.5/Ag content, 3.5%by mass).

It has also been found that contrary to the expectation that a decreasein the Ag content of a plated alloy film from the Ag content of theSn—Ag eutectic composition will incur a rise in the reflow temperature,the melting point of the alloy film does not increase significantly witha decrease in the Ag content, that is, it is not necessary tosignificantly raise the reflow temperature.

The present invention has been accomplished based on the above findings.Thus, the present invention provides a lead-free bump obtained byforming an Sn—Ag alloy film having a lower Ag content than that of anSn—Ag eutectic composition by plating and reflowing the plated alloyfilm.

The present invention also provide a method of forming a lead-free bumpcomprising: carrying out Sn—Ag alloy plating on a portion on which abump is formed while controlling the composition of a plating bath andelectrodeposition conditions so that a plated Sn—Ag alloy film having alower Ag content than that of the Sn—Ag eutectic composition is formed;and then reflowing the plated alloy film.

The present invention also provides a plating apparatus for forming alead-free bump, comprising: a plating vessel for containing a platingsolution having Ag ions and Sn ions; an anode; a holder for holding aworkpiece and feeding electricity to the workpiece; an electrodepositionpower source for feeding electricity to the anode and to the workpieceheld by the holder; a replenishment mechanism for replenishing theplating solution with Ag ions and Sn ions; an analyzer for monitoring Agions and Sn ions; and a control mechanism for controlling, on a basis ofanalytical information from the analyzer, an Ag content in a platedSn—Ag alloy film formed on a surface of the workpiece at a value lowerthan an Ag content of an Sn—Ag eutectic composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relationship between the concentrationratio of Ag ion to Sn ion in an alloy plating solution and the Agcontent in the plated film;

FIG. 2 is a diagram showing the relationship between the current densityin plating and the Ag content in the plated film, as observed when theplating is carried out by continuously applying a direct current;

FIG. 3 is a diagram showing a difference in the Ag content in a platedfilm between a plated film obtained by plating carried out bycontinuously applying a direct current (continuous direct currentplating) and a plated film obtained by plating carried out byintermittently applying a direct current (intermittent plating);

FIG. 4 is a diagram showing a plating apparatus according to anembodiment of the present invention;

FIG. 5 is a diagram showing an infrared oven for use in reflowing;

FIG. 6A is a diagram showing the sampling portions of sample beforereflowing which are used for quantitative analysis of Ag in Example 1,and FIG. 6B is a diagram showing the sample portions of samples afterreflowing which are used for quantitative analysis of Ag in Example 1;

FIG. 7A is an SEM photograph of a bump before reflowing obtained inExample 1, FIG. 7B is an SEM photograph of the bump shown in FIG. 7A butafter reflowing at 225° C., FIG. 7C is an SEM photograph of the bumpshown in FIG. 7A but after reflowing at 230° C., and FIG. 7D is an SEMphotograph of the bump shown in FIG. 7A but after reflowing at 238° C.;

FIG. 8 is an SEM photograph of the cross-section of a bump formed bysubjecting a plated Sn—Ag alloy film having an Ag content of 2.6% bymass to reflowing at 238° C.; and

FIG. 9 is an SEM photograph of the cross-section of a bump formed bysubjecting a plated Sn—Ag alloy film having an Ag content of 3.4% bymass to reflowing at 238° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The lead-free bump of the present invention can be obtained bydepositing a plated Sn—Ag alloy film by Sn—Ag alloy plating (hereinafterreferred to simply as “alloy plating”) carried out under such controlledelectrodeposition conditions that the plated Sn—Ag alloy film has an Agcontent which is lower than the Ag content of the Sn—Ag eutecticcomposition (i.e. 3.5 wt %), and then reflowing the plated Sn—Ag alloyfilm.

From the viewpoint of preventing the formation of voids, it may besufficient merely to control the Ag content in the plated alloy filmsuch that it is lower than the above-described upper limit. With the Agcontent ranging from 2.6 to 3.5% by mass, however, voids could be formedin some cases. Thus, in order to completely avoid voids, the Ag contentin the plated alloy film is preferably made not higher than 2.6% bymass.

Further, it is desirable from a practical viewpoint that the reflowtemperature be not so high (for example, the maximum reflow temperatureof not higher than 240° C.). For this purpose, it is preferred that thelower limit of the Ag content in the plated alloy film be made 1.6% bymass. Thus, in order to provide a practically desirable bump, the Agcontent in the plated alloy film is preferably made within the range of1.6 to 2.6% by mass.

Thus, according to the present invention, it is necessary to carry outplating while controlling the Ag content in the plated alloy film at avalue lower than 3.5% by mass, preferably from 1.6 to 2.6% by mass.

Further, with respect to the lead-free bump obtained by reflowing theplated alloy film as described above, the level of α-rays emitted fromthe surface of the plated alloy film is preferably not higher than 0.02cph/cm².

Lead has a plurality of isotopes, including natural radioactiveelements. The isotopes of lead are intermediate products or finalproducts in uranium or thorium decay series and emit α-rays in theirdecay processes. A-rays can act on semiconductor devices of asemiconductor integrated circuit and cause soft errors. Sn and otherelements also contain such natural radioactive elements though in aslight amount. Thus, a lead-free Sn—Ag bump also emits α-rays, and it isimportant to suppress the emission of α-rays at a low level. Bysuppressing the emission of α-rays from the surface of the lead-freebump at a level of not higher than 0.02 cph/cm², soft errors insemiconductor integrated circuit devices due to the influence of α-rayscan be prevented.

In general, the composition of deposited components in alloy plating isdetermined by the concentrations of the components in a plating solutionand the electrodeposition conditions. Also in alloy plating according tothe present invention, the Ag content in a plated alloy film can be madewithin the above-described range by adjusting the concentration ratio ofAg ion to Sn ion in an alloy plating solution, and controlling theelectrodeposition conditions. In particular, the Ag content in a platedalloy film can be controlled by (a) changing the electrodepositionconditions while keeping the concentration ratio of Ag ion to Sn ion ina plating bath constant, or (b) by changing the concentration ratio ofAg ion to Sn ion in a plating bath while keeping the electrodepositionconditions constant.

Though the alloy plating solution generally contains, besides the ionsof the metals forming an alloy, a complexing agent for stabilizing metalions, a brightening agent for making the surface of the plated filmbeautiful and/or other additive(s), the Ag content in the plated alloyfilm is primarily determined by the concentration ratio of Ag ion to Snion in the alloy plating bath. Thus, a plated alloy film having acontrolled Ag content can be obtained by finding a preferred range ofthe concentration ratio of Ag ion to Sn ion though experiments, andcarrying out plating while keeping the concentration ratio within thepreferred range. In fact, it has been confirmed that when carrying outalloy plating under fixed electrodeposition conditions, the Ag contentin a plated alloy film is proportional to the concentration ratio of Agion to Sn ion in a plating solution, as schematically shown in FIG. 1.

Accordingly, a plated alloy film having a controlled Ag content can beobtained by immersing a workpiece in an alloy plating solution with apredetermined concentration ratio of Ag ion to Sn ion, and carrying outplating under constant electrodeposition conditions. By reflowing theplated alloy film, a bump without voids can be obtained.

The following is an example of an alloy plating solution usable in thepresent invention;

-   -   Composition:    -   Sn ion (Sn²⁺) 10-100 g/L (preferably 35-50 g/L)    -   Ag ion (Ag⁺): 0.3-8 g/L (preferably 0.6-4 g/L)    -   Methanesulfonic acid: 100 g/L

It is known that, in alloy plating, the composition of depositedcomponents varies depending on the electrodeposition conditions. Also inalloy plating according to the present invention, the Ag content in aplated alloy film can be changed by changing the electrodepositionconditions.

Alloy plating according to the present invention may be carried out invarious manners using various types of electric currents, includingdirect current plating carried out by continuously applying a directcurrent, and an intermittent plating carried out by intermittentlyapplying a direct current with periodical rest periods.

In the case of direct current plating in which a direct current isapplied continuously during plating, the Ag content in the plated alloyfilm decreases with an increase in the current density, as schematicallyshown in FIG. 2. Preferable current conditions may be determinedexperimentally, and plating may be carried out while keeping thedetermined conditions. A preferred current density in direct currentplating is about 10 to 100 mA/cm².

In the case of intermittent plating in which a direct current is appliedintermittently with periodical rest periods, as schematically shown inFIG. 3, the plated alloy film has an Ag content which is different fromthe Ag content of a plated alloy film as obtained by applying the samedirect current continuously. Also with intermittent plating, preferableconditions, such as an applied voltage, the proportion of rest time,etc. may be determined experimentally, and plating may be carried outwhile keeping the determined conditions. A preferred current densityduring application of electric current is about 10-200 mA/cm², and apreferred rest time (zero current) is 1/10-1 of application time.

Though each applied voltage in the above two types of plating variesdepending upon conditions such as the intensity of current, theunderlying material, the thickness of plating, the plating solution, theanode used, etc., it is preferably about 1 to 5 V.

There is no particular limitation on an apparatus for carrying out theabove-described alloy plating, and a common dip type plating apparatusmay be employed. For a practical operation, however, it is desirable touse an apparatus which takes account of a jig structure configured tothe mechanical conditions of a workpiece, a stirring mechanism (paddlestructure) for supplying metal ions uniformly and rapidly to the entiresurface of a workpiece such as a wafer, the shape and size of a mask forequalizing the electric field distribution, a plating solutioncirculation system for removing foreign matter, preventing a change inthe quality of a plating solution and supplying metal ions uniformly andrapidly to the entire surface of a workpiece, etc.

Further, as described above, it is necessary to carry out alloy platingwhile adjusting the concentrations of Ag ions and Sn ions in a platingsolution, and controlling the electrodeposition conditions. It istherefore preferred to use a plating apparatus which is provided with areplenishment mechanism for replenishing an alloy plating solution withAg ions and Sn ions, an analyzer for monitoring Ag ions and Sn ions, anda control mechanical which, based on analytical information from theanalyzer, adjusts the concentrations of Ag ions and Sn ions in the alloyplating solution and/or controls the electrodeposition conditions. FIG.4 shows an example of such a plating apparatus.

In FIG. 4, reference numeral 1 denotes a plating apparatus, 2 denotes aplating vessel, 3 denotes an anode, 4 denotes a holder, 5 denotes aworkpiece, 6 denotes an electrodeposition power source, 7 denotesconductive wire, 8 a through 8 c denote a replenishment mechanism, 9denotes an analyzer, 10 denotes a control mechanism, 11 denotes anauto-sampler, 12 a through 12 c denote feed pumps, 13 denotes a shut-offvalve, and 14 denotes a discharge outlet.

The analyzer 9 periodically or continuously analyzes and monitors achange in the concentrations of Ag ions and Sn ions, as an index forcontrol of plating, which is due to consumption or loss of the ionsduring operation of the plating apparatus. An atomic absorptionspectrometry device, for example, may be used as the analyzer 9.

The control mechanism 10, which comprises, for example, a computer forcontrol, determines optimum replenishment amounts of Ag ions (solution),Sn ions (solution), etc. based on analytical information from theanalyzer 9, and actuates the feed bumps 12 a through 12 c, which areconnected to the replenishment mechanisms 8 a through 8 c, so as to addAg ions (solution) and Sn ions (solution) to a plating solution.

The replenishment mechanisms 8 a through 8 c, besides the portion forreplenishment of Ag ion solution and Sn ion solution, may additionallycomprise a portion for replenishing water for adjustment of thecomposition of plating solution, or an additive.

The anode 3, the holder 4 and the plating vessel 2 are each made of amaterial whose emission of α-rays is low so that the amount of α-raysemitted from the surface of a plated Sn—Ag alloy film formed by theplating apparatus 1 is made not higher than 0.02 cph/cm². Thus, theamount of natural radioactive elements taken in a bump of the platedalloy film formed by plating is made low, so that the amount of α-raysemitted from the bump can be made at such a low level. This effectivelysuppresses soft errors in semiconductor integrated circuit devices dueto the influence of α-rays.

The anode 3 may either be an insoluble anode or a soluble anode. It ispossible with an insoluble anode to continue using it without a change.When an Sn soluble anode is used as the anode 3, Sn ions can be suppliedfrom the anode to the plating solution during plating operation. Thisfacilitates control of the plating solution and reduces operation forreplenishment of metal ions.

The control mechanism 10 desirably controls the plating system underoptimum electrodeposition conditions (the above-described currentdensity and voltage application method) for a particular composition ofplating solution, and should make at least one of control of theelectrodeposition conditions and control of the concentrations of Agions and Sn ions by replenishment of the ions.

The actual deposition behavior of plating is influenced not only by theabove-described concentration ratio of Ag ion to Sn ion in an alloyplating solution and the electrodeposition conditions, but also by manyother factors. For example, the Ag content in a plated alloy film canvary depending on the type of the additive(s) added in the platingsolution. In most cases, however, the specific ingredients and theiramounts of an additive are undisclosed as the additive manufacturer'sknow-how.

For forming bumps according to the present inventions, therefore, it isnecessary to conduct experiments in advance with varying currentdensities in electroplating, voltage application methods and theconcentration ratios of Ag ions to Sn ion in an alloy plating solution,and measure the contents of Ag in the various bumps formed. Based on theresults of measurement, the optimum conditions for plating can bedetermined.

By carrying out plating under the optimum conditions thus determined, itbecomes possible to stabilize the composition of the plating solutionand stably form a bump having a desired Ag content.

The plated alloy film thus formed is then subjected to reflowing to forma bump. The reflowing is carried out by heating the plated alloy film inan inert gas atmosphere (e.g. nitrogen or argon atmosphere) using, forexample, the apparatus (infrared oven) shown in FIG. 5. In FIG. 5,reference numeral 20 denotes an infrared oven, 21 denotes a chamber, 22denotes a stage, 23 denotes a silica glass window, 24 denotes aninfrared lamp, and 25 denotes a workpiece.

The reflowing in this apparatus is carried out, for example, by settingthe workpiece 25, which has undergone alloy plating, in the chamber 21,allowing nitrogen gas to flow into the chamber 21 at a rate of about 8to 30 L/min to adequately carry out gas replacement, and then heatingthe workpiece 25 through the silica glass window 23 by the infrared lamp24.

The reflow temperature is important for the formation of bumps accordingto the present invention. A bump may be formed on a printed circuitboard, etc. Common electronic components are said to be heat-resistantto a temperature of about 240° C. The maximum temperature in the step ofreflowing a plated alloy film formed by alloy plating should thereforebe not higher than 240° C. Further, the melting point of an Sn—Ag solderis generally 221° C., and it is generally said that the minimumreflowable temperature is the melting point +10° C., and that the reflowtemperature must be maintained for 15 to 45 seconds. Taking suchrequirements into consideration, the temperature conditions uponreflowing may be exemplified by: 231° C. for 30 seconds with the maximumtemperature of 238° C.

The above-described lead-free bump of the present invention can beutilized, for example, as a ball-shaped bump on wiring pad in a mountingsubstrate.

In the formation of ball-shaped bumps, metal bond pads are first formed,and then a resist is applied on the substrate, with locations for bumpsbeing left, to form a resist pattern. Next, plated Sn—Ag alloy filmshaving a controlled Ag content are formed in the above-described manner.Thereafter, the resist is peeled off, and the alloy films are subjectedto reflowing at a predetermined reflow temperature.

Any one of the formation of the metal bond pads, the formation of theresist pattern and the removal of the resist may be carried out bycommon methods in the art.

Further, the lead-free bump of the present invention can be used to formwiring pads on a variety of semiconductor substrates. In particular, alead-free bump can be formed on a semiconductor substrate of asemiconductor device by the following steps (I) to (IV):

-   -   (I) Step of forming wiring pads on a semiconductor substrate of        a semiconductor device    -   (II) Step of forming a barrier metal on the wiring pads formed    -   (III) Step of forming an Sn—Ag plating on the barrier metal    -   (IV) Step of reflowing the Sn—Ag plating

The semiconductor device used in step (I) includes an integrated circuit(IC) and the like. A known barrier metal may be used as the barriermetal formed on the wiring pads in step (II).

The following examples are provided to illustrate the present inventionin greater detail and are not to be construed to be limiting theinvention in any manner.

EXAMPLE 1

(1) Preparation of Sn—Ag Bump:

A resist was applied to a thickness of 120 μm on a wafer in such amanner that a number of holes having an opening size of 100 μm areformed, thereby preparing a sample. The plating area of the sample was149.63 cm². Plating of the sample was carried out by the following stepsunder the following conditions.

(Plating Steps)

Degassing (10 min)→Pre-cleaning with 10% sulfuric acid (1 min)→Copperplating→Water-cleaning→Ni plating Water-cleaning→Sn—Ag alloy plating

(Plating Conditions)

(a) Cu Plating

Plating bath composition: Cu²⁺ 220 g/L H₂SO₄ 200 g/L HCl 5 mL/L Additive5 mL/L

Plating temperature: 25° C.

Stirring: mechanical stirring (paddle stirring speed 10 m/min)

Circulation of plating solution: flow rate 2.5 L/min

Electrode: copper anode, interpolar distance about 7.5 mm, anode mask ø250 mm

Cathode current density (total current): 5 A/dm² (7.48 A)

Plating thickness: 2 μm

(b) Ni Plating

Plating bath composition: Ni(NH₂SO₄).4H₂O 450 g/L H₃BO₃ 30 g/LNiCl₂.6H₂O 10 g/L Additive 2 mL/L

Plating temperature: 50° C.

Stirring: mechanical stirring (paddle stirring speed 10 m/min)

Circulation of plating solution: flow rate 2.5 L/min

Electrode: nickel anode, interpolar distance about 75 mm, anode mask ø250 mm

Cathode current density (total current): 3 A/dm² (4.49 A)

Plating thickness: 3 μm

(c) Sn—Ag Plating

plating bath composition: Sn²⁺ 40 g/L Ag⁺ 1.5 g/L methanesulfonic acid100 g/L Additive 10 g/L

[A 2:2:1 (weight ratio) mixture of polyoxyethylene surfactant, thioureaand cathechol]

Plating temperature: 25° C.

Stirring: mechanical stirring (paddle stirring speed 10 m/min)

Circulation of plating solution: flow rate 2.5 L/min

Electrode: titanium anode, interpolar distance about 7.5 mm, anode maskø 250 mm

Cathode current density (total current): 10 A/dm² (14.9A), directcurrent plating

Plating thickness: 140 μm

(2) Reflowing

After the plating described in (1) above, the resist was removed toexpose the plated portions. The plated portions were reflowed by usingan infrared oven as shown in FIG. 5. Temperature control of the infraredoven was carried out by placing a 2-inch silicon wafer with athermocouple embedded in the outermost layer at the center (temperaturemeasuring wafer made by SensArray Corporation) on the stage of theinfrared oven. The sample to be reflowed was placed near thethermocouple of the silicon wafer. After carrying out pre-heating at 150to 170° C. for 90 seconds, the sample was heated to a reflow temperaturein 30 seconds. The reflow temperature was from the minimum reflowabletemperature 231° C. to the maximum temperature 238° C. After maintainingthe reflow temperature for 30 seconds, the sample was cooled.

The interior of the infrared oven had been replaced with nitrogen gas,and the heating was carried out while flowing nitrogen gas at a rate of8 L/min. The infrared oven was employed because of its capability ofrapid heating and rapid cooling.

(3) Composition Analysis of Bump

The elemental composition of an Sn—Ag alloy bump was estimated in thefollowing manner. The bump was embedded in a resin, and the bump was cutto expose a cut surface. After polishing the cut surface, elementarymapping was performed by EPMA (Electron Probe Microanalysis). Further,quantitative analysis of Ag was carried out in three sectionalmicro-areas (l, c, r) as shown in FIGS. 6A and 6B, each having an areaof about 10 μm×10 μm, and the average of the measured valves wasdetermined as the Ag content of the bump.

A rough measurement of elemental composition is possible, without thenecessity of cutting a sample to expose a cut surface, by using a μfluorescent X-ray analyzer. Further, it is also possible to dissolve thebumps in an acid and analyze the compositional distribution in the waferby ICP-MS (Inductively Coupled Plasma Mass Spectrometer).

(4) Observation of the Shape of Bump

After the Sn—Ag alloy plating and the subsequent removal of the resist,the plated portions and the bumps after reflowing at varioustemperatures were observed under an SEM. FIG. 7A shows an SEM photographof a bump before reflowing, FIG. 7B shows an SEM photograph of a bumpafter reflowing at 225° C., FIG. 7C shows an SEM photograph of a bumpafter reflowing at 230° C., and FIG. 7D shows an SEM photograph of abump after reflowing at 238° C.

(5) Observation of Voids

A cut surface of a bump after reflowing at 238° C., was observed underan SEM. The observation was carried out after embedding the wafer in aresin, cutting the bump and polishing the cut surface. As a result, asshown in FIG. 8, no void was observed in a plated alloy film (bump)according to the present invention, having an Ag content of 2.6% bymass. In contrast, voids were formed in a bump of plated alloy filmhaving an Ag content of 3.4% by mass, as shown in FIG. 9.

EXAMPLE 2

Alloy plating was carried out with various proportions of Ag to thetotal metal in an alloy plating solution, various current densities uponplating and various current application methods, and the respectiveplated alloy films were subjected to reflowing at 238° C. For the bumpthus obtained, measurement of the Ag content and observation of theshape of bump and the presence of voids were carried out in the samemanner as in Example 1. The results are shown in table 1. TABLE 1Plating Plating conditions Voltage Bump Void Ball solution Currentdensity application Ag content present (x) formation Ag/Sn (%) (A/dm²)method (wt %) absent (∘) at 238° C. 4.3 3 DC 5.2 x ∘ 4.4 3 DC 6.4 x ∘4.4 20 CHOP 4.0 x ∘ 4.3 3 DC 7.7 x ∘ 4.3 20 CHOP 5.1 x ∘ 1.3 3 DC 1.8 ∘∘ 1.3 8 DC 0.9 ∘ x 1.3 20 CHOP 1.4 ∘ x 2.2 3 DC 3.4 x ∘ 2.2 20 CHOP 2.1∘ ∘ 4.1 3 DC 4.9 x ∘ 3.1 20 CHOP 2.9 ∘ ∘ 3.1 10 CHOP 3.6 x ∘ 3.1 3 DC5.0 x ∘ 3.2 3 DC 5.8 x ∘ 3.2 20 CHOP 2.7 x ∘ 2.3 3 DC 3.5 x ∘ 2.3 20CHOP 2.6 ∘ ∘Note:DC denotes direct current plating, and CHOP denotes intermittent plating

As apparent from the results shown in Table 1, voids are not formed in abump (plated alloy film) when the Ag content is 2.9% or lower.Especially when the Ag content is from 1.8 to 2.6%, the plated alloyfilm can be transformed into a ball at the maximum reflow temperature238° C. without formation of voids, providing a highly practicallead-free bump.

Lead-free bumps according to the present invention are free of voids,and are of a desirable ball shape that can be formed at a relatively lowreflow temperature. Further, those bumps do not contain lead, and thusdo not cause malfunction of an integrated circuit due to the emission ofα-rays nor environmental contamination.

Lead-free bumps according to the present invention can therefore bewidely used in surface mounting technology (SMT) of semiconductordevices, etc. and enable a reliable soldering despite their no inclusionof lead.

1. A method of forming a lead-free bump comprising: carrying out Sn—Ag alloy plating on a portion on which a bump is formed while controlling the composition of a plating bath and electrodeposition conditions so that a plated Sn—Ag alloy film having a lower Ag content than that of the Sn—Ag eutectic composition is formed; and then reflowing the plated alloy film.
 2. The method of forming a lead-free bump according to claim 1, wherein the Ag content in the plated Sn—Ag alloy film is 1.6 to 2.6% by mass.
 3. The method of forming a lead-free bump according to claim 2, wherein the maximum temperature of the reflowing the plated alloy film is not higher than 240° C.
 4. The method of forming a lead-free bump according to claim 1, wherein the control of the composition of the plating bath and the electrodeposition conditions is carried out by changing the electrodeposition conditions while keeping the ratio of concentration of Ag ion to Sn ion in the plating bath constant.
 5. The method of forming a lead-free bump according to claim 1, wherein the control of the composition of the plating bath and the electrodeposition conditions is carried out by changing the concentration ratio of Ag ion to Sn ion in the plating bath while keeping the electrodeposition conditions constant.
 6. A plating apparatus for forming a lead-free bump, comprising: a plating vessel for containing a plating solution having Ag ions and Sn ions; an anode; a holder for holding a workpiece and feeding electricity to the workpiece; an electrodeposition power source for feeding electricity to the anode and to the workpiece held by the holder; a replenishment mechanism for replenishing the plating solution with Ag ions and Sn ions; an analyzer for monitoring Ag ions and Sn ions; and a control mechanism for controlling, on a basis of analytical information from the analyzer, an Ag content in a plated Sn—Ag alloy film formed on a surface of the workpiece at a value lower than an Ag content of an Sn—Ag eutectic composition.
 7. The plating apparatus for forming a lead-free bump according to claim 6, wherein the Ag content in the plated Sn—Ag alloy film is controlled within the range of 1.6 to 2.6% by mass.
 8. The plating apparatus for forming a lead-free bump according to claim 6, wherein the Ag content in the plated Sn—Ag alloy film is controlled by adjustment of concentrations of Ag ions and Sn ions in the plating solution and/or change of electrodeposition conditions.
 9. The plating apparatus for forming a lead-free bump according to claim 6, wherein the anode, the holder and the plating vessel are made of materials whose amount of emission of α-rays is low so that an amount of α-rays emitted from a surface of the plated Sn—Ag alloy film is made not higher than 0.02 cph/cm².
 10. The plating apparatus for forming a lead-free bump according to claim 6, wherein the anode comprises an insoluble anode.
 11. The plating apparatus for forming a lead-free bump according to claim 6, wherein the anode comprises a soluble anode.
 12. The plating apparatus for forming a lead-free bump according to claim 11, wherein the anode comprises an Sn anode. 